1. Goal of HRW algorithm

NWC/GEO-HRW algorithm aims to provide detailed sets of Winds (Atmospheric Motion Vectors, AMVs) and Trajectories from up to seven MSG satellite series channels (HRVIS, VIS06, VIS08, IR108, IR120, WV062 and WV073), or from up to three GOES-N satellite series channels (VIS07, IR107 and WV065). These AMVs and Trajectories are calculated through the displacement of cloudiness features in successive images from all these channels, and of humidity features in successive images from the water vapour channels.

The AMVs and Trajectories can be calculated 24 hours a day, considering both "Nominal scan mode" conditions and "Rapid scan mode" conditions. The product includes pressure level information and a quality control flagging, giving some indication of its error in probabilistic terms. It has been developed by AEMET (the Spanish National Weather Service) in the framework of the EUMETSAT Satellite Application Facility on support to Nowcasting and Very short range forecasting (NWC SAF).

The AMVs and Trajectories can be calculated for up to two different scales at the same time: "Basic scale" and "Detailed scale". The corresponding dimension of the tracers to be tracked can be defined by the user, with a default value of 24 pixels for the "Basic scale". Many other processing parameters can also be configured, in particular the region and the MSG or GOES-N satellite channels to be processed for the AMV calculation.

This product can be useful in near real time applications including:

Nowcasting: Watch and warning of dangerous strong wind situations; Monitoring of the general flow, of convergence and divergence at low and high levels, or of small scale circulation and wind singularities.

Forecasting applications: Assimilation of HRW outputs in NWP models, or in applications where the real displacement of atmospheric structures is needed.

HRW output (as BUFR or netCDF bulletins) is similar to other products available at the WMO Global Telecommunications System. Winds, trajectories and the related parameters are calculated with a level 2 of processing. A level 3 of processing (as a grid interpolation of the winds or a meteorological analysis based on the data) is not included.

2. Summary description of HRW algorithm

Seven different steps are considered in the processing of HRW algorithm:

1. Initialization of data: next matrices are read or calculated for the start of the processing, by the NWC/GEO-NWCLIB library, for the running slot and region in which HRW algorithm is calculated:

Normalized reflectances for the MSG/GOES-N visible channel images, and brightness temperatures for the MSG/GOES-N infrared and water vapour channel images.

Gradient method: with a fast and efficient search of well defined cloud edges.

Tracer characteristics method: which covers holes in the search of tracers with Gradient method, with a longer but still reasonable computing time; two different tests are run based on finding a brightness threshold separating the cloudiness in front from the background, and finding a well defined shape of the feature to be tracked to avoid too linear elements.

An option inside the HRW algorithm process permits that new tracer centres for the running slot are defined at the integer final line/column positions of AMVs calculated previously at the same slot. This way, a set of “Persistent tracers” can be successively tracked in consecutive images, and “Trajectories” are defined through the progressive positions of these tracers throughout the time.

3. Tracer tracking and Wind calculation: One of two well known methods can be used for this process:

Euclidean distance.

Cross correlation (default option).

The tracking area in which tracers from a previous image are looked for in the current image, can be defined through the extrapolation of the linearly interpolated NWP wind guess for a quicker running of the algorithm. Nevertheless, this option is not used as default one to reduce the dependence from the NWP data. The best three tracking centres are kept, to perform a final selection step at the Quality Control.

4. Height assignment: Two different methods can be used:

Brightness temperature interpolation method: calculates the cloud top pressure and cloud base pressure, interpolating the brightness temperature of the corresponding satellite channel (MSG/IR120 or GOES-N/IR107 channel in the case of visible channels) to a NWP vertical temperature profile, and defines the AMV/Trajectory pressure through one of these two values considering the cloud type related to the tracer.

Cross Correlation Contribution (CCC) method: defines the AMV/Trajectory pressure, considering only the pressure of the pixels contributing most to the image correlation (default option, which is available only for Cross correlation tracking if NWC/GEO CT and CTTH outputs are available). A correction of the pressure level for cloudy AMVs/Trajectories is also implemented if so configured, considering the AMV/Trajectory Liquid water path/Ice water path (obtained from the corresponding NWC/GEO Cloud Microphysics output, and related to the depth of the cloud which is being tracked).

5. Quality Control: the “Quality Indicator method”, developed for the AMV calculation at EUMETSAT/MPEF, has been adapted for HRW algorithm. Several consistency tests are if possible computed, considering:

Two scale consistency test (only for the “Detailed scale”): vector consistency with simultaneous AMVs from the “Basic scale”.

Two individual “Quality Indices” (QIs, with and without contribution of the forecast consistency test) are calculated for each AMV/Trajectory, considering the normalized statistical fitting functions described by the method and the weighted sum of the partial consistency tests.

6. Orographic Flag: an “Orographic flag” is also calculated, related to the detection of land tracers, and tracers blocked by orography or related to orographic waves (for which the AMVs/Trajectories are not displacing with the corresponding atmospheric flow), with next possible values:

1: AMV wrongly located below the lowest representative pressure level in the vicinity (basically due to microphysics corrections in the AMV pressure). This AMV is always skipped.

2: Very important orographic influence is expected at the current position of the AMV.

3: Less important orographic influence is expected at the current position of the AMV.

4: Very important orographic influence is expected at any previous position of the AMV.

5: Less important orographic influence is expected at any previous position of the AMV.

6: No orographic influence is expected at any current or previous position of the AMV.

7. AMV Selection: a comparison is made among all AMVs calculated for the same tracer, when more than an AMV is available, for the final AMV selection for the given tracer (only one AMV per tracer can be kept). This comparison is made considering the behaviour of each AMV for each one of the following criteria:

From the AMV calculation process: tracking correlation.

From the Quality control process: temporal, spatial and forecast consistency tests; two scale consistency test also for the “Detailed scale”.

From the Orographic flag process: orographic flag.

Possible test values for each case are:

3: When the AMV is the best of all for the corresponding criterion.

2: When the AMV is slightly worse than others for the corresponding criterion.

1: When the AMV is rather worse than others for the corresponding criterion.

0: When the comparison could not be defined for the given AMV.

The AMV selection is based on the AMV with the best comparison tests. In case of draw, the AMV with the best forecast consistency test is selected, or else the AMV with the best tracking correlation.

3. Description of HRW inputs

MSG or GOES-N satellite data: Full Resolution MSG/HRIT or GOES-N/GVAR brightness temperatures for the infrared and water vapour channels to be used, and reflectances for the visible channels to be used, for the working region for the current and previous slot.

NWP data: Temperature fields and longitudinal and latitudinal component wind fields for as many as possible of the following pressure levels: 1000, 925, 850, 700, 500, 400, 300, 250, 200, 150, 100, 70, 50, 30, 10 hPa. Also geopotential fields in case the Orographic flag is to be calculated.

NWC/GEO Cloud Type (CT) and Cloud Top Temperature and Height (CTTH) output for the working region and slot (optionally, for a better height assignment).

NWC/GEO Cloud Microphysics (CMIC) output for the working region and slot (optionally, for a microphysics correction of the AMV pressure level, if CCC method is used for the height assignment). This option is only possible for MSG satellite series.

List of Tracers, Predecessor winds and Trajectories calculated at the previous slot (in case they are available).

4. Description of HRW outputs

Depending on configuration, AMVs and Trajectories are provided in the form of netCDF or BUFR bulletins, considering separately each processed region and each tracer scale used (“Basic scale” and “Detailed scale”).

HRW netCDF bulletins provide information on both AMVs and Trajectories, and are named S_NWC_HRW-WIND_*.nc.

Two different HRW BUFR AMV bulletin formats are also available: one similar to the format used by all previous versions of HRW algorithm (named S_NWC_HRW-WIND_*.bufr), and another one equivalent to the format used by the AMV bulletins provided by the EUMETSAT/MPEF (named S_NWC_HRW-WINDEUM_*.bufr). In the first case, which is the default option for HRW algorithm, if Trajectories are calculated, an additional BUFR bulletin (named S_NWC_HRW-TRAJ_*.bufr) is also provided with the content of the corresponding Trajectories.

The content of these netCDF or BUFR bulletins includes next parameters:

Type of tracer (Cloudy or Clear air tracer) and Height assignment method used.

Number of AMVs computed for the tracer (1 to 3).

Correlation and Quality tests: which or the partial QI consistency tests (two scale, temporal, spatial, forecast, orographic test) have been applied, and in case several AMVs were calculated for the tracer, if the selected one is better, slightly worse or rather worse than the other AMVs for the considered test.

Number of NWP levels used.

Orographic flag.

Cloud type.

MSG or GOES-N satellite channel used.

Correlation (if “Cross correlation” is used for the Tracer tracking).

Pressure error (if “CCC method” is used for the Height assignment).

5. Examples of HRW output visualization

Four different HRW algorithm configuration files (*.cfm) have been defined as default options.

Two of them are related to MSG satellite series, optimized for the running of HRW algorithm every 15 minutes (“Nominal scan” conditions) and every 5 minutes (“Rapid scan” conditions): safnwc_HRW.cfm.MSG15MIN and safnwc_HRW.cfm.MSG05MIN respectively.

The other two ones are related to GOES-N satellite series, optimized for the running of HRW algorithm every 15 minutes (in the “Continental United States” region) and every 30 minutes (in the “North America” region): safnwc_HRW.cfm.GOES15MIN and safnwc_HRW.cfm.GOES30MIN respectively.

Next figures show the output of the HRW algorithm AMVs for two specific examples, in which HRW algorithm has been run every 15 minutes with MSG-2 in the “European and Mediterranean region” and with GOES-13 satellites in the “Continental United States region”.